The present invention relates to a method of aligning at least two wave shaping elements of a measuring apparatus for interferometrically measuring a deviation of an optical surface from a target shape. Further, the invention relates to a method of measuring a deviation of an optical surface from a target shape, a method of manufacturing an optical element having an optical surface of an aspherical target shape. Additionally, the present invention relates to large aspheres or small aspheres having a large numerical aperture.
An optical surface to be measured can be the surface of an optical lens element or an optical mirror used in optical systems. Such optical systems can e.g. be configured as telescopes used in astronomy, and systems used for imaging structures, such as structures formed on a mask or a reticle, onto a radiation sensitive substrate, such as resist, by a lithographic method. The quality of such optical systems is substantially determined by the accuracy with which the optical surface can be machined or manufactured to have a target shape determined by a designer of the optical system. In such manufacturing it is necessary to compare the actual shape of the machined optical surface with its target shape and to determine differences between the machined and the target surfaces. The optical surface may then be further machined especially at those portions where differences between the machined and target surfaces exceed, for example, predefined thresholds.
Interferometric measuring apparatuses are commonly used for high precision measurements of optical surfaces. Examples of such apparatuses are disclosed in WO2005/114101. The entire content of this document is incorporated herein by reference.
An interferometric measuring apparatus for measuring a spherical optical surface typically includes a source of sufficiently coherent light and interferometer optics for generating a beam of measuring light incident on the surface to be tested, such that wave fronts of the measuring light have, at a position of the surface to be tested, the same shape as the target shape of the surface under test. In such a situation, the beam of measuring light is orthogonally incident on the surface under test, and is reflected therefrom to travel back towards the interferometer optics. Thereafter, the light of the measuring beam reflected from the surface under test is superimposed with light reflected from a reference surface and deviations between the shape of the surface under test and its target shape are determined from a resulting interference pattern.
While spherical wave fronts for testing spherical optical surfaces may be generated with a relatively high precision by conventional interferometer optics, more advanced optics, such as computer generated holograms (CGH's) are mostly necessary to generate beams of measuring light having an aspherical wave front such that the light is orthogonally incident at each location of an aspherical surface under test.
However, the size of CGH's available is limited, which in return limits the size of the aspherical optical surfaces testable with high accuracy, in particular if the optical surface is convex or raised.